This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions with coated carrier particles prepared by dry powder processes, and wherein supercritical fluids such as carbon dioxide is selected. In embodiments of the present invention, the carrier particles are comprised of a core with coating thereover, and which coating contains a surfactant to, for example, provide contrasting triboelectric carrier charging. In embodiments, the present invention relates to carrier particles and processes thereof, which processes comprise the preparation of polymer like poly(methacrylate) particles by supercritical polymerization in a medium, such as carbon dioxide, and wherein a surfactant, such as substituted polyacrylates, is selected for the reaction mixture. More specifically, in embodiments the present invention relates to the polymerization of a monomer like methyl methacrylate in supercritical carbon dioxide in the presence of a surfactant, and wherein small size carrier coatings, such as submicron to micron polymethylmethacrylate (PMMA) carrier coatings with a size, for example, of from about 0.05 to about 5 microns, and more specifically, from about 0.05 to about 1 micron. The surfactant selected for the aforementioned polymerization is believed to stabilize the particles during polymerization, and such surfactant enables the preselection of the triboelectric charge on the carrier particles comprised of a core and the polymers obtained with the invention processes. The carrier polymers thus can be comprised of a polymer like PMMA, and thereover a controlled amount of contrasting triboelectric surfactant. Moreover, in another aspect of the present invention the carrier particles are prepared by a dry coating process wherein a mixture polymer obtained with covered surfactant is applied to a carrier core enabling insulating particles with relatively constant conductivity parameters; and also wherein the triboelectric charge on the carrier can be varied depending on the coating selected. Specifically, therefore, with the carrier compositions and process of the present invention there can be formulated developers with selected triboelectric charging characteristics and/or conductivity values in a number of different combinations. Developer compositions comprised of the carrier particles prepared by the dry coating process of the present invention are useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging and printing processes. Additionally, developer compositions comprised of substantially insulating carrier particles prepared in accordance with the process of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core. With the processes of the present invention, costly washing and drying steps can be avoided or minimized, environmental concerns such as the discarding of waste solvent can be eliminated or minimized, carrier morphologies can be controlled, and the carrier coating can include conductive components, such as carbon black, metal oxides like tin oxide, and the like therein in an amount, for example, of from about 20 to about 45 weight percent to obtain carrier particles with certain conductivities.
The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that enable continued constant developed images of high quality and excellent resolution.
Additionally, carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may be comprised of various cores, including steel, with a coating thereover of fluoropolymers; and terpolymers of styrene, methacrylate, and silane compounds. Many of the commercial carrier coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of 20 minutes to about 120 minutes, enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent, the disclosure of which has been totally incorporated herein by reference, are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics; and further, with regard to the '387 patent, in many situations carrier and developer mixtures with only specific triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated. With the invention of the present application, the conductivity of the resulting carrier particles are substantially constant, and moreover, the triboelectric values can be selected to vary significantly, for example, from less than -15 microcoulombs per gram to greater than -70 microcoulombs per gram, depending on the polymer mixture selected for affecting the coating process. Also, illustrated in Creatura et al. U.S. Pat. Nos. 4,937,166, and 4,935,326, the disclosures of which are totally incorporated herein by reference, is a carrier composition comprised of a core with a coating thereover comprised of a mixture of first and second polymers that are not in close proximity thereto in the triboelectric series, and which carrier can be prepared by dry coating processes.
With further reference to the prior art, carriers obtained by applying insulating resinous coatings to porous metallic carrier cores using solution coating techniques are undesirable from many viewpoints. For example, the coating material will usually reside in the pores of the carrier cores, rather than at the surfaces thereof, and therefore, is not available for triboelectric charging when the coated carrier particles are mixed with finely divided toner particles. Attempts to resolve this problem by increasing the carrier coating weights, for example, to as much as 3 percent or greater to provide an effective triboelectric coating to the carrier particles necessarily involves handling excessive quantities of solvents, and further, usually these processes result in low product yields. Also, solution coated carrier particles when combined and mixed with finely divided toner particles provide in some instances triboelectric charging values which are too low for many uses.
Thus, for example, there can be formulated in accordance with the invention of the present application developers with conductivities of from about 10.sup.-6 mho (cm).sup.-1 to about 10.sup.-17 mho (cm).sup.-1 as determined in a magnetic brush conducting cell, and triboelectric charging values of from about a -8 to about -80 microcoulombs per gram on the carrier particles as determined by the known Faraday Cage technique. Thus, the developers of the present invention can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, maintaining the same coating weight on the carrier particles and changing the polymer coating ratios. Similarly, there can be formulated developer compositions wherein constant triboelectric charging values are achieved and the conductivities are altered by retaining the polymer ratio coating constant and modifying the coating weight for the carrier particles.
In copending patent application U.S. Ser. No. 314,745, the disclosure of which is totally incorporated herein by reference, there is illustrated a process comprising subjecting a toner comprised of resin and pigment to a particle size reduction in an organic fluid; accomplishing supercritical extraction thereof with, for example, carbon dioxide; and isolating said toner.
Also, reference is made to the following copending patent applications filed concurrently herewith, the disclosures of which are totally incorporated herein by reference, U.S. Ser. No. 08/415,278 entitled Carrier Powder Supercritical Polymers, U.S. Ser. No. 08/415,261 entitled Carrier Coatings and Processes, U.S. Ser. No. 08/415,261 entitled Coated Carrier Particles, and U.S. Ser. No. 08/415,384 entitled Supercritical Polymerization Processes.